Drug-delivery endovascular stent and method for treating restenosis

ABSTRACT

An intravascular stent and method for inhibiting restenosis, following vascular injury, is disclosed. The stent has an expandable, linked-filament body and a drug-release coating formed on the stent-body filaments, for contacting the vessel injury site when the stent is placed in-situ in an expanded condition. The coating releases, for a period of at least 4 weeks, a restenosis-inhibiting amount of the macrocyclic triene immunosuppressive compound everolimus. The stent, when used to treat a vascular injury, gives good protection against clinical restenosis, even when the extent of vascular injury involves vessel overstretching by more than 30% diameter. Also disclosed is a stent having a drug-release coating composed of (i) 10 and 60 weight percent poly-dl-lactide polymer substrate and (ii) 40-90 weight percent of an anti-restenosis compound, and a polymer undercoat having a thickness of between 1-5 microns.

This application is a continuation-in-part of U.S. application Ser. No.10/133,814 filed Apr. 24, 2002, incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an endovascular drug-delivery stent andto a method for treating restenosis.

BACKGROUND OF THE INVENTION

A stent is a type of endovascular implant, usually generally tubular inshape, typically having a lattice, connected-wire tubular constructionwhich is expandable to be permanently inserted into a blood vessel toprovide mechanical support to the vessel and to maintain or re-establisha flow channel during or following angioplasty. The support structure ofthe stent is designed to prevent early collapse of a vessel that hasbeen weakened and damaged by angioplasty. Insertion of stents has beenshown to prevent negative remodeling and spasm of the vessel whilehealing of the damaged vessel wall proceeds over a period of months.

During the healing process, inflammation caused by angioplasty and stentimplant injury often causes smooth muscle cell proliferation andregrowth inside the stent, thus partially closing the flow channel, andthereby reducing or eliminating the beneficial effect of theangioplasty/stenting procedure. This process is called restenosis. Bloodclots may also form inside of the newly implanted stent due to thethrombotic nature of the stent surfaces, even when biocompatiblematerials are used to form the stent.

While large blood clots may not form during the angioplasty procedureitself or immediately post-procedure due to the current practice ofinjecting powerful anti-platelet drugs into the blood circulation, somethrombosis is always present, at least on a microscopic level on stentsurfaces, and it is thought to play a significant role in the earlystages of restenosis by establishing a biocompatible matrix on thesurfaces of the stent whereupon smooth muscle cells may subsequentlyattach and multiply.

Stent coatings are known which contain bioactive agents that aredesigned to reduce or eliminate thrombosis or restenosis. Such bioactiveagents may be dispersed or dissolved in either a bio-durable orbio-erodable polymer matrix that is attached to the surface of the stentwires prior to implant. After implantation, the bioactive agent diffusesout of the polymer matrix and preferably into the surrounding tissueover a period lasting at least four weeks, and in some cases up to oneyear or longer, ideally matching the time course of restenosis, smoothmuscle cell proliferation, thrombosis or a combination thereof.

If the polymer is bioerodable, in addition to release of the drugthrough the process of diffusion, the bioactive agent may also bereleased as the polymer degrades or dissolves, making the agent morereadily available to the surrounding tissue environment. Bioerodablestents and biodurable stents are known where the outer surfaces or eventhe entire bulk of polymer material is porous. For example, PCTPublication No. WO 99/07308, which is commonly owned with the presentapplication, discloses such stents, and is expressly incorporated byreference herein. When bioerodable polymers are used as drug deliverycoatings, porosity is variously claimed to aid tissue ingrowth, make theerosion of the polymer more predictable, or to regulate or enhance therate of drug release, as, for example, disclosed in U.S. Pat. Nos.6,099,562, 5,873,904, 5,342,348, 5,873,904, 5,707,385, 5,824,048,5,527,337, 5,306,286, and 6,013,853.

Heparin, as well as other anti-platelet or anti-thrombolytic surfacecoatings, are known which are chemically bound to the surface of thestent to reduce thrombosis. A heparinized surface is known to interferewith the blood-clotting cascade in humans, preventing attachment ofplatelets (a precursor to thrombin) on the stent surface. Stents havebeen described which include both a heparin surface and an active agentstored inside of a coating (see U.S. Pat. Nos. 6,231,600 and 5,288,711,for example).

A variety of agents specifically claimed to inhibit smooth muscle-cellproliferation, and thus inhibit restenosis, have been proposed forrelease from endovascular stents. As examples, U.S. Pat. No. 6,159,488describes the use of a quinazolinone derivative; U.S. Pat. No.6,171,609, the use of taxol, and U.S. Pat. No. 5,716,981, the use ofpaclitaxel, a cytotoxic agent thought to be the active ingredient in theagent taxol. The metal silver is cited in U.S. Pat. No. 5,873,904.Tranilast, a membrane stabilizing agent thought to haveanti-inflammatory properties is disclosed in U.S. Pat. No. 5,733,327.

More recently, rapamycin, an immunosuppressant reported to suppress bothsmooth muscle cell and endothelial cell growth, has been shown to haveimproved effectiveness against restenosis, when delivered from a polymercoating on a stent. See, for example, U.S. Pat. Nos. 5,288,711 and6,153,252. Also, in PCT Publication No. WO 97/35575, the macrocyclictriene immunosuppressive compound everolimus and related compounds havebeen proposed for treating restenosis, via systemic delivery.

Ideally, a compound selected for inhibiting restenosis, by drug releasefrom a stent, should have three properties. First, because the stentshould have a low profile, meaning a thin polymer matrix, the compoundshould be sufficiently active to produce a continuous therapeutic dosefor a minimum period of 4-8 weeks when released from a thin polymercoating. Secondly, the compound should be effective, at a low dose, ininhibiting smooth muscle cell proliferation. Finally, endothelial cellswhich line the inside surface of the vessel lumen are normally damagedby the process of angioplasty and/or stenting. The compound should allowfor regrowth of endothelial cells inside the vessel lumen, to provide areturn to vessel homeostasis and to promote normal and criticalinteractions between the vessel walls and blood flowing through thevessel.

SUMMARY OF THE INVENTION

The invention includes, in one aspect, a method for inhibitingrestenosis at a vascular injury site. The method comprises delivering tothe vascular injury site an endovascular stent having an open-latticestructure formed of linked filaments, and carried on the one or morefilaments, a drug-release coating. The drug release coating has athickness of between 3-30 microns and is composed of (i) 20 and 70weight percent polymer substrate and (ii) 30-80 weight percentmacrocyclic triene compound having the form:

where R is CH₂—CH₂—OH. The stent is expanded at the vascular injury siteto bring the drug-release coating in contact with the vessel at theinjury site, where the coating is effective to release an amount of thecompound to inhibit restenosis at the site.

In one embodiment, the stent body is a metal-filament structure, and thepolymer substrate in the coating is selected from the group consistingof polymethylmethacrylate, ethylene vinyl alcohol, poly-lactidepolymers, ε-caprolactone, ethyl vinyl acetate, polyvinyl alcohol, andpolyethylene oxide. In one preferred embodiment, the polymer substratein the coating is formed of poly-dl-lactide having a thickness between3-20 microns and the compound is present in the coating at an initialconcentration of between 35 and 80 weight percent of coating.

In another embodiment, the stent for use in the method further includesa polymer undercoat disposed between the filaments of the stent body andthe drug-release coating. Exemplary polymers for the undercoat includeethylene vinyl alcohol, parylast, silicone, a fluoropolymer, andparylene. In an exemplary stent, a parylene polymer undercoat having athickness of between 1-3 microns is deposited, the underlayer disposedbetween the filaments of the stent body and a poly-dl-lactide coatingsubstrate.

The compound can be present in the coating in an amount between 50% and75% by weight. In a preferred embodiment, the drug release coating has adrug-to-polymer ratio of 54% drug and 46% polymer by weight.

The polymer coating on the stent can further include a bioactive agentselected from the group consisting of an antiplatelet agent, afibrinolytic agent, and a thrombolytic agent.

In another aspect, the invention includes an improvement in a method forinhibiting restenosis at a vascular injury site, by placement at thesite an intravascular stent designed to release a macrocyclic trienecompound over an extended period. The improvement comprises employing asthe macrocyclic triene compound, a compound having the formula:

where R is CH₂—CH₂—OH.

In one embodiment, the improvement is for use where the vascular injuryis produced during an angiographic procedure in which a vessel region isoverstretched at least 30% in diameter.

In another embodiment, the compound is carried on the stent in adrug-release coating composed of a polymer substrate and having between30-80 weight percent of the compound.

In yet another aspect, the invention includes an endovascular stent forplacement at a vascular injury site, for inhibiting restenosis at thesite. The stent is comprised of a body having an open-lattice structureformed of linked filaments, and carried on the one or more filaments, adrug-release coating having a thickness of between 3-30 microns, andcomposed of (i) 20 and 70 weight percent polymer substrate and (ii)30-80 weight percent macrocyclic triene compound having the form:

where R is CH₂—CH₂—OH. The stent is expandable from a contractedcondition in which the stent can be delivered to a vascular injury sitevia catheter, and an expanded condition in which the stent coating canbe placed in contact with the vessel at the injury site, where thecoating is effective to release an amount of the compound to inhibitrestenosis at the site.

In one embodiment of this aspect, the stent body is a metal-filamentstructure, and the polymer substrate in the coating is selected from thegroup consisting of polymethylmethacrylate, ethylene vinyl alcohol,poly-lactide polymers, ε-caprolactone, ethyl vinyl acetate, polyvinylalcohol, and polyethylene oxide. In one exemplary embodiment, thepolymer substrate in the coating is formed of poly-dl-lactide having athickness between 3-20 microns and the compound is present in thecoating at an initial concentration of between 35 and 80 weight percentof coating.

The stent, in another embodiment, includes a parylene polymer undercoathaving a thickness of between 1-3 microns, disposed between thefilaments of the stent body and a poly-dl-lactide coating substrate. Inthis embodiment, the compound can be present at an initial concentrationof between 50 and 75 weight percent of coating.

More generally, the stent can comprise a polymer undercoat disposedbetween the filaments of the stent body and said drug-release coating.Exemplary materials for the polymer undercoat include ethylene vinylalcohol, parylast, silicone, a fluoropolymer, and parylene.

In another embodiment, the stent coating further includes a secondbioactive agent selected from the group consisting of antiplateletagents, fibrinolytic agents, and thrombolytic agents.

In another embodiment, the stent body filaments are comprised of abiodegradable polymer.

The invention also contemplates an apparatus for delivery of a stent asdescribed above, the apparatus comprised of a catheter suitable fordelivery of the stent and the stent.

These and other objects and features of the invention will become morefully apparent when the following detailed description of the inventionis read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate an endovascular stent having a metal-filamentbody, and formed in accordance with one embodiment of the presentinvention, showing the stent in its contracted (FIG. 1) and expanded(FIG. 2) conditions;

FIG. 3 is an enlarged cross-sectional view of a coated metal filament inthe stent of FIG. 1;

FIG. 4 is an enlarged cross-sectional view of coated polymer stent;

FIGS. 5A and 5B are schematic illustrations of a polymer coating methodsuitable for use in producing the coated stent of the invention;

FIGS. 6A and 6B are plots showing release of everolimus from stentsconstructed in accordance with the invention;

FIG. 7 is a cross-sectional view of a stent in the invention deployed ata vascular site;

FIGS. 8A-8C are histological sections of a vessel 28 days afterimplantation of a bare-metal stent;

FIGS. 9A-9C are histological sections of a vessel 28 days afterimplantation of a metal-filament stent with a polymer coating;

FIGS. 10A-10C and 11A-11C are histological sections of a vessel 28 daysafter implantation of a metal-filament stent with a polymer coatingcontaining everolimus;

FIG. 12 is an enlarged histological section of a vessel seen with afilament of the stent employed in FIGS. 10A-10C, which been overgrown bynew tissue forming a healed vessel wall;

FIG. 13 is a plot of area of stenosis at 28 days post-implant, as afunction of injury score, with a variety of different stents, includingthose constructed in accordance with the invention; and

FIG. 14 shows a correlation plot between injury score (Y axis) and B/A(balloon/artery) ratio at time of stent implantation.

DETAILED DESCRIPTION OF THE INVENTION

I. Endovascular Stent

FIGS. 1 and 2 show a stent 20 constructed in accordance with theinvention, in the stent's contracted and expanded states, respectively.The stent includes a structural member or body 22 and an outer coatingfor holding and releasing an anti-restenosis compound, as will bedescribed further below with reference to FIGS. 3 and 4.

A. Stent Body

In the embodiment shown, the stent body is formed of a plurality oflinked tubular members by filaments, such as members 24, 26. Each memberhas an expandable zig-zag, sawtooth, or sinusoidal wave structure. Themembers are linked by axial links, such as links 28, 30 joining thepeaks and troughs of adjacent members. As can be appreciated, thisconstruction allows the stent to be expanded from a contractedcondition, shown in FIG. 1, to an expanded condition, shown in FIG. 2,with little or no change in the length of the stent. At the same time,the relatively infrequent links between peaks and troughs of adjacenttubular members allows the stent to accommodate bending. This featuremay be particularly important when the stent is being delivered to avascular site in its contracted state, in or on a catheter. The stenthas a typical contracted-state diameter (FIG. 1) of between 0.5-2 mm,more preferably 0.71 to 1.65 mm, and a length of between 5-100 mm. Inits expanded state, shown in FIG. 2, the stent diameter is at leasttwice and up to 8-9 times that of the stent in its contracted state.Thus, a stent with a contracted diameter of between 0.7 to 1.5 mm mayexpand radially to a selected expanded state of between 2-8 mm or more.

Stents having this general stent-body architecture of linked, expandabletubular members are known, for example, as described in PCT PublicationNo. WO 99/07308, which is commonly owned with the present application,and which is expressly incorporated by reference herein. Furtherexamples are described in U.S. Pat. Nos. 6,190,406, 6,042,606,5,860,999, 6,129,755, or 5,902,317, which patents are incorporated byreference herein. Alternatively, the structural member in the stent mayhave a continuous helical ribbon construction, that is, where the stentbody is formed of a single continuous ribbon-like coil. The basicrequirement of the stent body is that it be expandable, upon deploymentat a vascular injury site, and that it is suitable for receiving adrug-containing coating on its outer surface, for delivering drugcontained in the coating into the vessel wall (i.e. medial, adventitial,and endothelial layers of tissue) lining the vascular target site.Preferably, the body also has a lattice or open structure, allowingendothelial cell wall ingrowth “through” the stent from outside toinside.

B. Stent Coatings

According to an important feature of the invention, the stent filamentsare coated with a drug-release coating composed of a polymer matrix andan anti-restenosis compound (active compound) distributed within thematrix for release from the stent over an at least a several weekperiod, typically 4-8 weeks, and optionally over a 2-3-month period ormore.

FIG. 3 shows, in enlarged sectional view, a stent filament 24 having acoating 32 that covers the filament completely on all sides, that is, ontop (the filament side forming the outer surface of the stent body)bottom (the filament side forming the interior surface of the stent) andthe opposing filament sides. As will be discussed further below, thecoating has a thickness typically between 3 and 30 microns, depending onthe nature of the polymer matrix material forming the coating and therelative amounts of polymer matrix and active compound. Ideally, thecoating is made as thin as possible, e.g., 15 microns or less, tominimize the stent profile in the vessel at the injury site.

The coating should also be relatively uniform in thickness across theupper (outer) surfaces, to promote even distribution of released drug atthe target site. Methods for producing a relatively even coatingthickness on stent filaments are discussed below in Section II.

Also shown in FIG. 3 is a polymer underlayer 34 disposed between thestent filament and the coating. The purpose of the underlayer is to helpbond the coating to the stent-body filaments, that is, to help stabilizethe coating on the filaments. As will be seen below, this function isparticularly valuable where the coating is formed of a polymer substratecontaining a high percentage of anti-restenosis compound, e.g. between35-80 weight percent compound. One exemplary underlayer polymer isparylene, and in one embodiment parylene is used in conjunction with apolymer substrate formed of bioerodable (poly-dl-lactide). Othersuitable polymer underlayers are ethylene vinyl alcohol (EVOH),paryLAST™, silicone, TEFLON™ and other fluoropolymers, that may bedeposited on the metal stent surfaces by plasma-coating or other coatingor deposition processes. The underlayer has a typical thickness between1-5 microns.

The polymer forming the substrate may be any biocompatible polymermaterial from which entrapped compound can be released by diffusionand/or released by erosion of the polymer matrix. Two well-knownnon-erodable polymers for the coating substrate arepolymethylmethacrylate and ethylene vinyl alcohol. Methods for preparingthese polymers in a form suitable for application to a stent body aredescribed for example, in US 2001/0027340A1 and WO00145763A1,incorporated herein by reference. In general, the limit of drug additionto the polymers is about in the range of 20-40 weight percent.

Bioerodable polymers, particularly poly-dl-lactide polymer, are alsosuitable for coating substrate material. In one general embodiment, ofthe invention, the coating is a bioerodable poly-dl-lactide polymersubstrate, poly-dl-lactic acid polymer, that may contain up to 80% bydry weight of the active compound distributed within the polymersubstrate. More generally, the coating contains 35-80% dry weight activecompound and 20-65% percent by dry weight of the polymer. Exemplarycoatings include 25-50% dry weight polymer matrix and 50-75 weightpercent active compound. The polymer is formulated with the activecompound for deposition on the stent filaments as detailed in Section IIbelow.

A variety of anti-restenosis compounds may be employed in theembodiment, including anti-proliferative agents, such as taxol,antisense compounds, doxorubicin, and most particularly, macrocyclictriene immunosuppressive compounds having the general structureindicated below. The latter class of compounds, and their synthesis, aredescribed, for example in U.S. Pat. Nos. 4,650,803, 5,288,711,5,516,781, 5,665,772 and 6,153,252, in PCT Publication No. WO 97/35575,and in published U.S. patent applications U.S. Pat. No. 6,273,913B1,60/176,086, 20000212/17, and 2001002935/A1, all of which areincorporated herein by reference. An exemplary macrocyclic trieneimmunosuppressive compound has the form:

where (i) R is H or CH₂—X—OH, and X is CH₂. This compound is known aseverolimus. Where R═H, the compound is known as rapamycin.

One preferred coating is formed of 25-50 weight percent poly-dl-lactidepolymer substrate, and 50-75 weight percent macrocyclic trieneimmunosuppressant compound, having a coating thickness of between 3-15microns. The underlayer is formed of parylene, and has a thicknessbetween 1-5 microns. This embodiment typically contains an amount ofcompound equal to about 15 micrograms drug/mm of stent length.

In another exemplary embodiment, the coating is formed of 15-35 weightpercent of an erodable or non-erodable polymer substrate, and 65-85weight percent of the macrocyclic triene compound. The coating thicknessis preferably 10-30 microns, and the stent may include a 1-5 micronpolymer underlayer, e.g., parylene underlayer. This embodiment typicallycontains an amount of compound equal to about 15 micrograms drug/mm ofstent length.

The coating may additionally include a second bioactive agent effectiveto minimize blood-related events, such as clotting, that may bestimulated by the original vascular injury, the presence of the stent;or to improve vascular healing at the injury site. Exemplary secondagents include anti-platelet, fibrinolytic, or thrombolytic agents insoluble crystalline form. Exemplary anti-platelet, fibrinolytic, orthrombolytic agents are heparin, aspirin, hirudin, ticlopidine,eptifibatide, urokinase, streptokinase, tissue plasminogen activator(TPA), or mixtures thereof. The amount of second-agent included in thestent coating will be determined by the period over which the agent willneed to provide therapeutic benefit. Typically, the agent will bebeneficial over the first few days after vascular injury and stentimplantation, although for some agents, longer period of release of theagent will be required.

The second agent may be included in the coating formulation that isapplied to the stent-body filaments, according to known methods.

C. Bioerodable Stent

In another general embodiment, both the stent body and polymer coatingare formed of a bioerodable polymer, allowing complete resorption of thestent over time. The stent preferably is an expandable coiled stenthaving a helical-ribbon filament forming the stent body (not shown).Self-expandable coil stents are described in U.S. Pat. No. 4,990,155 forimplantation into blood vessels and are incorporated herein byreference.

A coiled stent, may be formed using a preform with the final expandeddiameter of the preform specified to be slightly larger than theinternal lumen size of the blood vessel to be treated with the coil (3.5mm OD±1 mm would be common for a coronary artery). More generally, thestent may be formed by molding, in its expanded shape, and placed in itscontracted state by twisting around the stent's long axis or forcing thestent radially into a contracted condition for delivery to the bloodvessel when mounted on the tip of a catheter. The stent has a totalthickness preferably between about 100 and 1000 microns, and a totallength of between 0.4 and 10 cm. In fact, an important advantage of abioerodable stent of this type is that relatively long stents, e.g.,over 3 cm in length, can be readily delivered and deployed at a vascularinjury site.

Methods for forming balloon-expandable stents formed of a knitted,bioerodable polymer filament such as poly-l-lactide have been reported(U.S. Pat. No. 6,080,177). A version of the device has also been adaptedto release drugs (U.S. Pat. No. 5,733,327).

A preferred polymer material for forming the stent is poly-l- orpoly-dl-lactide (U.S. Pat. No. 6,080,177). As indicated above, the stentbody and coating may be formed integrally as a single expandablefilament stent having anti-restenosis compound contained throughout.Alternatively, a bioerodable coating may be applied to a preformedbioerodable body, as detailed in Section II below. In the latter case,the stent body may be formed of one bioerodable polymer, such aspoly-l-lactide polymer, and the coating from a second polymer, such aspoly-dl-lactide polymer. The coating, if applied to a preformed stent,may have substantially the same compositional and thicknesscharacteristics described above.

FIG. 4 shows a cross section of a filament, e.g., helical ribbon, in abioerodable stent of the type just described, having separately formedpolymer body and coating. The figure shows an internal polymer stentfilament 36 coated on all sides with a bioerodable coating 38. Anexemplary coating is formed of poly-dl-lactide and contains between20-40 weight percent anti-restenosis drug, such as the macrocyclictriene immunosuppressant compound everolimus, and 60-80 weight percentpolymer substrate. In another general embodiment, the coating contains45-75 weight percent compound, and 25-55 weight percent polymer matrix.Other types of anti-restenosis compounds, such as listed above, may beemployed in either embodiment.

The bioerodable stent has the unique advantage of treating the entirevessel with one device, either in conjunction with pre-dilatation of thevessel with balloon angioplasty if large obstructions are present, or asa prophylactic implant in patients of high risk of developingsignificant future blockages. Since the stent is fully biodegradable, itdoes not affect the patient's chances for later uncomplicated surgery onthe vessel, as does a “full metal jacket,” i.e., a string of drugeluting stents containing metal substrates.

A secondary agent, such as indicated above, may be incorporated into thecoating for release from the coating over a desired time period afterimplantation. Alternatively, if a secondary agent is used, it may beincorporated into the stent-body filament if the coating applied to thestent body does not cover the interior surfaces of the stent body. Thecoating methods described below in Section II with respect to ametal-filament stent body are also suitable for use in coating apolymer-filament stent body.

II. Stent Coating Methods

Referring now to FIGS. 5A and 5B, schematic illustrations of a stentcoating process according to the invention are shown. A polymer solution40 is made by dissolving a polymer in a compatible solvent. At least oneanti-restenosis compound, and if desired, a secondary agent, is added tothe solution, either as a suspension or in solution using the samesolvent or a different solvent. The completed mixture is placed in apressurizable reservoir 42. Connected to the reservoir is a fluidpressurization pump 44.

The pressurization pump may be any source of pressure capable of urgingthe solvent mixture to move at a programmed rate through a solutiondelivery tube 46. The pressure pump 44 is under the control of amicrocontroller (not shown), as is well known in the field of precisiondispensing systems. For example, such a microcontroller may comprise4-Axis Dispensing Robot Model numbers I&J500-R and I&J750-R availablefrom I&J Fisnar Inc, of Fair Lawn, N.J., which are controllable throughan RS-232C communications interface by a personal computer, or precisiondispensing systems such as Automove A-400, from Asymtek, of Carlsbad,Calif. A suitable software program for controlling an RS232C interfacemay comprise the Fluidmove system, also available from Asymtek Inc,Carlsbad, Calif.

Attached to reservoir 42, for example, at the bottom of the reservoir,is a solution delivery tube 48 for delivery of the solvent mixture tothe surface of the stent. The pressurizable reservoir 42 and deliverytube 48 are mounted to a moveable support (not shown) which is capableof moving the solvent delivery tube in small steps such as 0.2 mm perstep, or continuously, along the longitudinal axis of the stent as isillustrated by arrow X1. The moveable support for pressurizablereservoir 42 and delivery tube 46 is also capable of moving the tip(distal end) of the delivery tube closer to the microfilament surface orup away from the microfilament surface in small steps as shown by arrowY1.

The uncoated stent is gripped by a rotating chuck contacting the innersurface of the stent at least one end. Axial rotation of the stent canbe accomplished in small degree steps, such as 0.5 degree per step, toreposition the uppermost surface of the stent structure for coating bythe delivery tube by attachment of a stepper motor to the chuck as iswell known in the art. If desirable, the stent can be rotatedcontinuously. The method of precisely positioning a low volume fluiddelivery device is well known in the field of X-Y-Z solvent dispensingsystems and can be incorporated into the present invention.

The action of the fluid pressurizing pump, X1 and Y1 positioning of thefluid delivery tube, and R1 positioning of the stent are typicallycoordinated by a digital controller and computer software program, suchthat the precisely required amount of solution is deposited whereverdesired on the surfaces of the stent, whereupon the solvent is allowedto escape, leaving a hardened coating of polymer and agent on the stentsurfaces. Typically, the viscosity of the solvent mixture is prepared byvarying the amount of solvent, and it ranges from 2 centipoise to 2000centipoise, and typically can be 300 to 700 centipoise. Alternatively,the delivery tube can be held at a fixed position and, in addition tothe rotation movement, the stent is moved along its longitudinaldirection to accomplish the coating process.

The X-Y-Z positioning table and moveable support may be purchased fromI&J. Fisnar. The solution delivery tube preferred dimensions arepreferably between 18-28 gauge stainless steel hypotubes mounted to asuitable locking connector. Such delivery tubes may be obtained from EFDInc of East Providence, R.I. See EFD's selection guide for SpecialPurpose Tips. The preferred tips are reorder #'s 5118-¼-B through5121-¼-B “Burr-free passivated stainless steel tips with ¼” length forfast point-to-point dispensing of particle-filled or thick materials”,reorder #'s 51150VAL-B “Oval stainless steel tips apply thick pastes,sealants, and epoxies in flat ribbon deposits”, and reorder #'s5121-TLC-B through 5125-TLC-B “Resists clogging of cyanoacrylates andprovides additional deposit control for low viscosity fluids. Crimpedand Teflon lined”. A disposable pressurizable solution reservoir is alsoavailable from EFD, stock number 1000Y5148 through 1000Y 5152F. Analternate tip for use with the invention is a glass micro-capillary withan I.D. of about 0.0005 to 0.002 inch, such as about 0.001 inch, whichis available from VWR Catalog No. 15401-560 “Microhematocrit Tubes”, 60mm length, I.D. 0.5-0.6 mm.

The tubes are further drawn under a Bunsen burner to achieve the desiredI.D. for precise application of the polymer/drug/solvent mixture. Theprogrammable microcontroller to operate the stepper motor, and XYZ tableis available from Asymtek, Inc. It is within the scope of the inventionto use more than one of the fluid dispensing tube types working inconcert to form the coating, or alternately to use more than onemoveable solution reservoir equipped with different tips, or containingdifferent viscosity solutions or different chemical makeup of themultiple solutions in the same process to form the coating. The chuckand stepper motor system may be purchased from Edmund Scientific ofBarrington, N.J.

Typically, as described above, the coating is applied directly onto theoutside support surface(s) of the stent, and may or may not cover theentire or a portion(s) of the inside surface(s) of the stent dependingon how control is applied to the above described coating system of thepresent invention, as illustrated in FIGS. 5A and 5B. The latter figureshows application of a coating material 52 to top and side regions of afilament 50. Alternatively, the coating or coating mixture can also beapplied directly onto the inside surface of the stent. A thin deliverytip may penetrate through one or more of the cut out areas (i.e.windows) in the wall of the stent structure, and thereby apply thecoating mixture directly onto the inside surfaces at desired areas. Inthis method, it is possible to apply different coating materials havingdifferent drug components to outer and inner sides of the filaments. Forexample, the coating on the outer filament surfaces could contain ananti-restenosis compound, and the coating of the inner filamentsurfaces, one of the above secondary agents, such an anti-thrombotic oranti-clotting compound. If the stent has a large enough diameter, a thin“L-shaped” delivery tip can be inserted into the stent open ends alongthe longitudinal axis of the stent for the purpose of applying coatingto the inside surfaces.

The polymer for use in the invention includes, but is not limited to,poly(d,l-lactic acid), poly(l-lactic acid), poly(d-lactic acid),ethylene vinyl alcohol (EVOH), e-caprolactone, ethylvinyl hydroxylatedacetate (EVA), polyvinyl alcohol (PVA), polyethylene oxides (PEO), andco-polymers thereof and mixtures thereof, dissolved in a suitablesolvent or solvent mixture Exemplary solvents include chloroform, ethylacetate, acetone, and the like, and mixtures of these solvents. Thesepolymers all have a history of safe and low inflammatory use in thesystemic circulation.

A non-polymer coating such as everolimus which has been ionically boundto the metal stent surface can also be used in the present invention.

Using the coating system as described, it has been discovered that it isfeasible to coat all of the top, side, and inside surfaces of the stent.By the careful selection of a suitable ratio of solvent to polymer, theviscosity of the solution can be adjusted such that some of the solutionwill migrate down the sides of the strut and actually inhabit the bottomsurface before solidifying, as shown in FIG. 5B. By controlling thedwell time of the delivery tube close to the edge of the stent, theamount of polymer coating the edges or bottom of the stent can beincreased or reduced. In the embodiment illustrated in FIG. 3, anunderlayer 34 of pure polymer and solvent is applied to the stentsurfaces 24 first using the coating system of the invention and thesolvent is allowed to evaporate. Then a second layer of polymer 32 isapplied containing the bioactive agent.

As noted above, a secondary agent may be incorporated into the polymermixture. As an example, heparin in crystalline form may be incorporatedinto the coating. The heparin crystals are micronized to a particle sizeof approximately 1-5 microns and added in suspension to the polymersolution. Suitable forms of heparin are those of crystalline form thatexhibit bioactivity in mammalian hosts when applied according to theprocess of the invention, including heparin salts (i.e. sodium heparinand low molecular weight forms of heparin and their salts). Upondeployment of the drug delivering stent 62 into vessel wall 60, as seenin FIG. 7, the heparin crystals near the surface of the coating of curedpolymer 66 begin to dissolve, increasing the porosity of the polymer. Asthe polymer slowly dissolves, more heparin and bioactive agent arereleased in a controlled manner, as indicated by arrows 68.

It should be appreciated however, with reference to FIG. 7, that it isnot always desirable to coat the inside surfaces of the stent (indicatedat 64 in FIG. 7). For example, coating the inside surface of the stentincreases the crimped delivery profile of the device, making it lessmaneuverable in small vessels. And, after implantation, the insidesurfaces are directly washed by the flow of blood through the stent,causing any drug released on the inside surface to be lost to thesystemic circulation. Therefore, in the embodiments shown in FIGS. 3 and4, the bulk of the cured polymer and agent is deployed on the outsidecircumference of the stent supports, and secondarily on the sides. In apreferred embodiment, only a minimum amount of polymer and agent isapplied on the inside surfaces of the stent. If desired, it is alsopossible to have at least a portion of the inside surfaces of the stentuncoated or exposed.

Further, the coating of FIGS. 3 and 4, may be placed onto the stentfilament surfaces in a selective manner. The depth of the coated sectionmay correspond to the volume of bioactive coating to be available forpresentation to the tissue. It may be advantageous to restrict thecoating from certain areas, such as those which could incur high strainlevels during stent deployment.

A uniform underlayer may be first placed on the stent surface to promoteadhesion of the coating that contains the bioactive agent, and/or tohelp stabilize the polymer coating on the stent. The primer coat may beapplied by using any of the methods as already known in the art, or bythe precision dispensing system of the invention. It is also within thescope of the invention to apply a primer coat using a different polymermaterial, such as parylene (poly(dichloro-para-xylylene)) or a parylenederivative or analog, or any other material which exhibits good adhesionto both the base metal substrate and the coating which contains thebioactive agent. Parylene (poly(dichloro-para-xylylene)) may bedeposited via plasma deposition or vapor deposition techniques, as iswell known in the art and described, for example, in U.S. Pat. No.6,299,604, the portions of which relating to plasma and vapor depositionof parylene are incorporated by reference herein. In one embodiment ofthe present invention, islands or a layer of a coating containingheparin are formed on inside surface(s) of a stent and ananti-proliferation coating containing the drugs of the present inventionas described above is formed on outside surface(s) of the stent.

Where it is desired to form a coating with a high drug/polymer substrateratio, e.g., where the drug constitutes 40-80 weight percent of thecoating on a metal stent substrate, it is advantageous to form anunderlayer on the stent filaments to stabilize and firmly attach thecoating to the substrate. The underlayer may be further processed, priorto deposition of the coating material, by swelling in a suitablesolvent, e.g., acetone, chloroform, ethyl acetate, xylene, or mixturesthereof. This approach is described in Example 5 for preparing a stenthaving a high ratio of everolimus to poly-dl-lactide.

Here a parylene underlayer is formed on the stent filaments by plasmadeposition, and the underlayer then allowed to swell in xylene prior tofinal deposition of the coating material. The method was effective inproducing coating containing 50% drug in one case and 75% drug inanother case in a poly-dl-lactide polymer substrate, in a coating havinga thickness of only 5-10 microns.

III. Methods of Use and Performance Characteristics

This section describes vascular treatment methods in accordance with theinvention, and the performance characteristics of stents constructed inaccordance with the invention.

A. Methods

The methods of the invention are designed to minimize the risk and/orextent of restenosis in a patient who has received localized vascularinjury, or who is at risk of vascular occlusion. Typically the vascularinjury is produced during an angiographic procedure to open a partiallyoccluded vessel, such as a coronary or peripheral vascular artery. Inthe angiographic procedure, a balloon catheter is placed at theocclusion site, and a distal-end balloon is inflated and deflated one ormore times to force the occluded vessel open. This vessel expansion,particularly involving surface trauma at the vessel wall where plaquemay be dislodged, often produces enough localized injury that the vesselresponds over time by cell proliferation and reocclusion. Notsurprisingly, the occurrence or severity of restenosis is often relatedto the extent of vessel stretching involved in the angiographicprocedure. Particularly where overstretching is 35% or more, restenosisoccurs with high frequency and often with substantial severity, i.e.,vascular occlusion.

In practicing the present invention, the stent is placed in itscontracted state typically at the distal end of a catheter, eitherwithin the catheter lumen, or in a contracted state on a distal endballoon. The distal catheter end is then guided to the injury site, orthe site of potential occlusion, and released from the catheter, e.g.,by using a trip wire to release the stent into the site, if the stent isself-expanding, or by expanding the stent on a balloon by ballooninflation, until the stent contacts the vessel walls, in effect,implanting the stent into the tissue wall at the site.

Once deployed at the site, the stent begins to release active compoundinto the cells lining the vascular site, to inhibit cellularproliferation. FIG. 6A shows everolimus release kinetics from two stentsconstructed in accordance with the invention, each having anapproximately 10 micron thick coating (closed squares). Drug-releasekinetics were obtained by submerging the stent in a 25% ethanolsolution, which greatly accelerates rate of drug release from the stentcoating. The graphs indicate the type of drug release kinetics that canbe expected in vivo, but over a much longer time scale.

FIG. 6B shows drug release of everolimus from coatings of the presentinvention on metal stent substrates. The upper set of curves show drugrelease where the coating has been applied directly to the metalsurface. The lower set of curves (showing slower release) were obtainedby applying an underlayer or primer coat of parylene to the metal stentsurface, followed by coating of the surface with the coating system ofthe invention. As seen, the primer increases the mechanical adhesion ofthe coating to the sent surface, resulting in slower breakdown of thebioerodeable coating and slower release of drug. Such a configuration isuseful where it desired to have a strongly attached stent coating whichcan withstand repeated abrasions during tortuous maneuvering of the drugeluting stent inside the guide catheter and/or vessel, and/or where itis desired to slow down the drug release for extended treatment of theathersclerosis disease process at the implant site followingimplantation of the device.

FIG. 7 shows in cross-section, a vascular region 60 having an implantedstent 62 whose coated filaments, such as filament 64 with coating 66,are seen in cross section. The figure illustrates the release ofanti-restenosis compound from each filament region into the surroundingvascular wall region. Over time, the smooth muscle cells forming thevascular wall begin to grow into and through the lattice or helicalopenings in the stent, ultimately forming a continuous inner cell layerthat engulfs the stent on both sides. If the stent implantation has beensuccessful, the extent of late vascular occlusion at the site will beless than 50%, that is, the cross-sectional diameter of flow channelremaining inside the vessel will be at least 50% of expanded stentdiameter at time of implant.

Trials in a swine restenosis animal model as generally described bySchwartz et al. (“Restenosis After Balloon Angioplasty-A PracticalProliferative Model in Porcine Coronary Arteries”, Circulation 82:(6)2190-2200, December 1990.) demonstrate the ability of the stent of thisinvention to limit the extent of restenosis, and the advantages of thestent over currently proposed and tested stents, particularly in casesof severe vascular injury, i.e., greater than 35% vessel stretching. Thestudies are summarized in Example 4.

Briefly, the studies compare the extent of restenosis at 28 daysfollowing stent implantation, in bare metal stents, polymer-coatedstents, and polymer-coated stents containing high or low concentrationsof sirolimus (rapamycin) and everolimus.

Table 1 in Example 4 shows that both rapamycin (Rapa-high or Rapa-low)and everolimus stents (C-high or C-low) greatly reduced levels ofrestenosis, with the smallest amount of restenosis being observed in thehigh-dose everolimus stent. Similar results were obtained in studies onanimals with low injury (Table 2).

FIGS. 8A-8C are examples of stent cross-sections of neointimal formationat 28 days in a bare metal S-Stent (available from BiosensorsInternational Inc, Newport Beach, Calif.). FIGS. 9A-9C are examples ofneointimal formation in a polymer-coated (no drug) S-Stent; and FIGS.10A-10C and 11A-11C of neointimal formation in everolimus/polymer coatedstents. In general, the vessels with everolimus-coated stent treatmentappeared to be well-healed with a well established endothelial layer,evidence of complete healing and vessel homeostasis at 28 days. FIG. 12is an example of vessel cross-section at 91× magnification showinghealing and establishment of an endothelial layer on the inside of thevessel lumen at 28 days post implant.

The photographs indicate that the most favorable combination forelimination of restenosis at 28 days is the C-high, or C-Ulightformulation (see Example 4), which contained 325 microgram and 275microgram dosages of everolimus, respectively, on a 18.7 mm lengthstent. The data predicts a 50% reduction in restenosis compared to acurrently marketed bare metal stent (the S-Stent) at 28 days follow-upin outbred juvenile swine. The data also shows that the drug everolimusis better than, or at least equivalent to the 180 microgram dosage ofsirolimus on the same stent/polymer delivery platform. These results aresupported by morphometric analysis (Example 4).

FIG. 13 is a plot showing “best fit” linear regression curves of thechosen dosings of agents in polymers, coated on the S-Stent, relatinginjury score to area stenosis at follow-up. Area stenosis is an accurateindicator of neointimal formation which is determined by morphometricanalysis. As can be seen from this chart, the high everolimus stent wasthe only coating in the group of samples tested that exhibited anegative slope vs. increasing injury score. This analysis suggests thatthe C-high coating may be capable of controlling restenosis in aninjured coronary artery which is virtually independent of injury score.None of the other coating formulations tried exhibited this uniquecharacteristic.

FIG. 14 shows the relationship between balloon overstretch of thevessel, as measured by balloon/artery ration (B/A Ratio), and vesselinjury, in the animal experiment. This data shows that use of anover-expanded angioplasty balloon to create a high controlled vesselinjury is a reasonably accurate method of creating a predictable andknown vascular injury in the porcine model.

From the foregoing, it can be seen how various objects and features ofthe invention are met. In one aspect, the invention provides abioerodable stent coating with high drug/polymer ratios, e.g., 40-80%drug by weight. This feature allows continuous delivery of ananti-restenosis compound over an extended period from a low-profilestent. At the same time, the total amount of polymer breakdowncomponents such as lactide and lactic acid released during bioerosion isrelatively small, minimizing possible side effects, such as irritation,that may result from bioerosion of the stent coating.

In another aspect, the invention provides an improved method fortreating or inhibiting restenosis. The method, which involves a novelcombination of macrocyclic triene immunosuppressant compound in a stentpolymer coating, provides at least the effectiveness against restenosisas the best stent in the prior art, but with the added advantage overthe prior art that the efficacy of the method appears to be independentof the extent of injury, and the method may offer a greater degree ofendothelialization of the stented vessel.

Finally, the method provides a completely bioerodable stent that has theadvantageous features just mentioned and the more design flexibilitythan a metal-body stent, particularly in total stent length and futureoperability on the treated vessel.

The following examples illustrate various aspects of the making andusing the stent invention herein. They are not intended to limit thescope of the invention.

Example 1 Preparation of Everolimus Step A. Synthesis of2-(t-butyldimethylsilyl)oxyethanol(TBS glycol)

154 mL of dry THF and 1.88 g NaH are stirred under in a nitrogenatmosphere in a 500 mL round bottom flask condenser. 4.4 mL dry ethyleneglycol are added into the flask, resulting in a large precipitate after45 minutes of stirring. 11.8 g tert-butyldimethylsilyl chloride is addedto the flask and vigorous stirring is continued for 45 minutes. Theresulting mixture is poured into 950 mL ethylether. The ether is washedwith 420 mL brine and solution is dried with sodium sulfate. The productis concentrated by evaporation of the ether in vacuo and purified byflash chromatography using a 27×5.75 cm column charged with silica gelusing a hexanes/Et₂O (75:25v/v) solvent system. The product is stored at0° C.

Step B. Synthesis of 2-(t-butyldimethylsilyl)oxyethyl triflate (TBSglycol Trif)

4.22 g TBS glycol and 5.2 g 2,6-lutidine are combined in a double-necked100 mL flask with condenser under nitrogen with vigorous stirring. 10.74g of trifluoromethane sulfonic anhydride is added slowly to the flaskover a period of 35-45 minutes to yield a yellowish-brown solution. Thereaction is then quenched by adding 1 mL of brine, and the solutionwashed 5 times in 100 mL brine to a final pH value of between 6-7. Thesolution is dried using sodium sulfate, and concentrated by evaporationof the methylene chloride in vacuo. The product is purified using aflash chromatography column of approximately 24×3 cm packed with silicagel using hexane/Et₂O (85:15v/v) solvent system, then stored at 0° C.

Step C. Synthesis of 40-O-[2-(t-butyldimethylsilyl)oxy]ethyl-rapamycin(TBS Rap)

400 mg rapamycin, 10 mL of toluene, and 1.9 mL 2,6-lutidine are combinedand stirred in a 50 mL flask maintained at 55-57° C. In a separate 3 mLseptum vial, 940 μL 2,6-lutidine is added to 1 mL toluene, followed byaddition of 2.47 g TBS glycol Trif. The contents of the vial are addedto the 50 mL flask and the reaction allowed to proceed for 1.5 hourswith stirring. 480 μL 2,6-lutidine plus an additional 1.236 g TBS glycolTrif is added to the reaction flask. Stirring is continued for anadditional hour. Finally, a second portion of 480 μL 2,6-lutidine and1.236 g TBS glycol Trif is added to the mixture, and the mixture isallowed to stir for an additional 1-1.5 hours. The resulting brownsolution is poured through a porous glass filter-using vacuum. Thecrystal like precipitate is washed with toluene until all color has beenremoved. The filtrate is then washed with 60 mL saturated NaHCO₃solution twice and then washed again with brine. The resulting solutionis dried with sodium sulfate and concentrated in vacua A small quantityof a hexanes/EtOAc (40:60 v/v) solvent is used to dissolve the product,and purification is achieved using a 33×2 cm flash chromatography columnpacked with silica gel, and developed with the same solvent. The solventis removed in vacuo and the product stored at 5° C.

Step D. Synthesis process of 40-0-(2-hydroxyl)ethyl-rapamycin(everolimus)

A pyrex glass dish (150×75 mm) is filled with ice and placed on astirring plate. A small amount of water is added to provide an iceslurry. 60-65 mg of TBS-Rap is first dissolved in a glass vial by adding8 mL methanol. 0.8 mL 1N HCI is added to the vial, the solution isstirred for 45 minutes and then neutralized by adding 3 mL aqueoussaturated NaHCO₃. 5 mL brine is added to the solution, followed with 20mL EtoAc, resulting in the formation of two phases. After mixing of thephases, a separatory funnel is used to draw off the aqueous layer. Theremaining solvent is washed with brine to a final pH of 6-7, and driedwith sodium sulfate. The sodium sulfate is removed using a porous glassfilter, and the solvent removed in vacuo. The resulting concentrate isdissolved in EtoAc/methanol (97:3) and then purified using in a 23×2 cmflash chromatography column packed with silica gel, and developed usingthe same solvent system. The solvent is removed in vacuo and the productstored at 5° C.

Example 2 Preparation of Stent Containing Everolimus in aPoly-dl-Lactide Coating

100 mg poly (dl-lactide) was dissolved into 2 mL acetone at roomtemperature. 5 mg everolimus was placed in a vial and 400 μL lactidesolution added. A microprocessor-controlled syringe pump was used toprecision dispense 10 μL of the drug containing lactide solution to thestent strut top surfaces. Evaporation of the solvent resulted in auniform, drug containing single polymer layer on the stent.

A 15 μL volume was used in a similar manner to coat the stent top andside strut surfaces, resulting in a single layer coating on the stentstrut top and sides.

Example 3 In Vitro Drug Release from Stent Containing Everolimus in aPoly-dl-Lactide Coating

In vitro drug release was conducted by placing the coated stents into 2mL pH 7.4 phosphate buffered saline solution containing 25% ETOH, andpreserved with 0.05% (w/v) sodium azide and maintained at 37° C.Sampling was periodically conducted by withdrawing the total buffervolume for drug measurement while replacing solution with a similarvolume of fresh buffer (infinite sink). FIG. 6 illustrates drug releasefrom two similar stents coated with a single polymer layermicrodispensed in this manner.

Example 4 Animal Implant Tests

A. QCA Results of Safety and Dose-Finding Studies in Swine Rationale:

It was reasoned that the most challenging treatment condition for thedrug eluting stent is a severely injured vessel, as it is known that thedegree of restenosis (neointimal formation) increases directly withextent of vessel injury. Experiments were conducted in pigs, and asubstantial number of the vessels which were the target of drug-coatedstent implants were seriously injured (averaging approximately 36%overstretch injury of the vessel) using an angioplasty balloon. Thiscaused severe tearing and stretching of the vessel's intimal and mediallayers, resulting in exuberant restenosis at 28 days post implant. Inthis way, it was possible to assess the relative effectiveness ofvarious dosings of drug, and drug to polymer weight ratios on the samemetal stent/polymer platform for reduction of restenosis at 28 dayspost-implant.

DEFINITIONS

1. Bare stent: An 18.7 mm bare metal stent of a corrugated ring design(i.e. a currently marketed “S-Stent” as manufactured by BiosensorsIntl., Inc).

2. C-high: An 18.7 mm long stent carrying 325 micrograms of everolimusin a PDLA (poly-dl-lactate) polymer coating.

3. C-low: An 18.7 mm long stent carrying 180 micrograms of everolimus ina PDLA polymer coating.

4. Rap-high: An 18.7 mm long stent carrying 325 micrograms of sirolimusin a PLA polymer coating.

5. Rap-low: An 18.7 mm long stent carrying 180 micrograms of sirolimusin a PDLA polymer coating.

6. C-Ulight: An 18.7 mm long stent carrying 275 micrograms of Everolimusin an ultrathin coating of PDLA polymer (37% drug to polymer weightratio).

7. C-Ulow: An 18.7 mm long stent carrying 180 micrograms of Everolimusor equivalent in an ultrathin coating of PDLA polymer (37% drug topolymer weight ratio).

8. Polymer stent: An 18.7 mm S-Stent stent covered by PDLA polymercoating only.

9. B/A is the final inflated balloon-to-artery ratio, an indication ofthe extent of overstretching of the vessel.

10. Mean Lumen Loss (MLL)-Average of 3 measurements taken inside thestent internal lumen at time of implant minus average of 3 measurementsat follow-up angiography indicates the amount of neointima that hasformed inside the stent.

Methods:

Drug-eluting stents using a metal wire-mesh scaffold of a corrugatedring design (i.e. S-Stent) and polymer coating were implanted inout-bred juvenile swine (alternately Yucatan Minipigs for implantstudies lasting longer than 28 days), using different dosings of eitherthe drug everolimus or the drug sirolimus. At time of implant,Quantitative Coronary Angiography (QCA) was performed to measure thediameter of the vessels both before and after stent implantation. At 28days, or longer when specified in the table below, the animals wereagain subjected to QCA in the area of the stent, prior to euthanization.

Following euthanasia of animals according to approved protocols, thehearts were removed from the animals and pressurized formaldehydesolution was infused into the coronary arteries. The coronary segmentscontaining the stents were then surgically removed from the surface ofthe heart and subsequently fixed in acrylic plastic blocks fortransverse sectioning with a diamond saw. Sections of the acrylicmaterial, 50 μm in thickness, containing cross-sections of the vesselslocated proximally, center, and distally were then optically polishedand mounted to microscope slides.

A microscope containing a digital camera was used to generate highresolution images of the vessel cross-sections which had been mounted toslides. The images were subjected to histomorphometric analysis by theprocedure as follows:

A computerized imaging system Image Pro Plus 4.0 through an A.G. Heinzeslide microscope for a PC-based system was used for histomorphometricmeasurements of:

1. The mean cross sectional area and lumen thickness (area circumscribedby the intima/neointimal-luminal border); neointimal (area between thelumen and the internal elastic lamina, IEL, and when the IEL wasmissing, the area between the lumen and the remnants of media or theexternal elastic lamina, EEL); media (area between the IEL and EEL);vessel size (area circumscribed by the EEL but excluding the adventitialarea); and adventitia area (area between the periadventitial tissues,adipose tissue and myocardium, and EEL).

2. The injury score. To quantify the degree of vascular injury, a scorebased on the amount and length of tear of the different wall structureswas used. The degree of injury was calculated as follows:

-   -   0=intact IEL    -   1=ruptured IEL with exposure to superficial medial layers (minor        injury)    -   2=ruptured IEL with exposure to deeper medial layers (medial        dissection)    -   3=ruptured EEL with exposure to the adventitia.

The following table shows the results of the QCA analysis (measurementsof mean late loss due to restenosis) at follow-up QCA. The data in thetables below under column heading “Neo-intimal area” report the resultsof morphometric analysis of stents and vessels removed from the pigs atfollow-up (f/u):

TABLE 1 Results of “high injury” experiment Mean Neo- B/A Lumen IntimaDevice Ratio Days Loss, mm Area Description (avg) f/u (avg) (mm²) Stentnumbers Bare Metal 1.33 28 1.69 5.89 31, 39, 40, 45, 47, 50 StentPolymer 1.36 28 2.10 5.82 32, 41, 43, 48, 51, 60 Coated Rapa-high 1.3928 1.07 3.75 42, 44, 49, 65, 69, 73 Rapa-low 1.42 28 0.99 2.80 52, 56,61, 64, 68, 72 C-high 1.37 28 0.84 3.54 54, 55, 59, 63 C-low 1.36 281.54 3.41 53, 57, 58, 62, 66, 70, 74 C-Uhigh 1.36 28 0.85 2.97 67, 75,92, 103

B. Low-Injury Studies

To further determine which dosage of everolimus would be best in alightly injured vessel, more typical of the patient with uncomplicatedcoronary disease and a single denovo lesion, the everolimus-elutingstents were implanted to create moderate to low overstretch injury(approximately 15%). Farm swine were used for a 30 day experiment, andadult Yucatan minipigs were implanted for a 3 month safety study. Theangiographic results were as follows:

TABLE 2 QCA Results of “low injury” experiments Neo- Days Mean IntimaDevice B/A post Lumen Area Description ratio implant Loss (mm²) Stentnumbers Bare Metal 1.14 28 0.95 2.89 20, 22, 26, 29 Stent Bare Metal1.13 90 76, 80, 84, 87, 91 Stent C-Uhigh 1.15 28 0.60 2.14 94, 96, 98,102 C-Ulow 1.09 28 0.49 2.26 93, 95, 97, 100, 101 C-Uhigh 1.15 90 77,81, 85, 86, 90

The above data predict that with either the C-Ulow or C-Uhigh doses ofeverolimus will produce a 45-48% reduction in neointimal formation in alow to moderately injured vessel.

C. Morphometric Analysis

The total cross-sectional area inside each stent, and cross-sectionalarea of new tissue (neo-intima) that had formed inside the stent weremeasured by computer, and the percent area stenosis computed. Theaverage vessel injury score, neo-intimal area, and % area stenosis foreach formulation of drug and polymer, averaging three slices per stent,is shown in the table below.

TABLE 3 Results of “high injury” experiment Neo- Intimal Device InjuryDays Area % Area Description Score f/u (mm²) Stenosis Stent numbers BareMetal 1.9 28 5.89 0.72 31, 39, 40, 45, 47, 50 Stent Polymer 2.11 28 5.820.70 32, 41, 43, 48, 51, 60 Coated Rapa-high 2.10 28 3.75 0.55 42, 44,49, 65, 69, 73 Rapa-low 1.90 28 2.80 0.43 52, 56, 61, 64, 68, 72 C-high1.89 28 3.54 0.38 54, 55, 59, 63 C-low 2.1 28 3.41 0.53 53, 57, 58, 62,66, 70, 74 C-Uhigh 2.13 28 2.97 0.45 67, 75, 92, 103

Morphometric analysis is considered a highly accurate method ofmeasuring in-stent restenosis in the pig coronary model. In the highinjury model, the C-High formulation produced the lowest amounts ofneointima formation in the ‘High Injury’ experiment at 28 days, however,the C-Uhigh had the highest injury score of the group, and still manageda very low % Area Stenosis of 0.45. Therefore, the data independentlyconfirm the findings of the QCA analysis, and supports the choice ofC-Uhigh as the preferred formulation for human trials.

D. Histological Analysis

The slides for the C-Uhigh and Sirolimus Low were submitted to anexperienced cardiac pathologist, who reviewed the vessel cross-sectionsfor evidence of inflammation, fibrin, and endothelialization of thenewly healed vessel lumen. No difference was found between thehistological changes caused by the sirolimus and everolimus elutingstents. In general, the vessels appeared to be well-healed with a wellestablished endothelial layer, evidence of complete healing and vesselhomeostasis at 28 days. FIG. 12 is an example of vessel cross-section at91× magnification showing healing and establishment of an endotheliallayer on the inside of the vessel lumen at 28 days post-implant.

E. Comparison to Published Results

Carter et al. have published results of sirolimus-coated stents usingthe Palmaz Schatz metal stent in swine. A table comparing the publishedresults of Carter to Biosensors' experimental results is shown below:

TABLE 4 Mean Std Neointima Vessel Late Loss Deviation Cross-SectionalOverstretch (mm) (mm) Area (mm²) DEVICE DESCRIPTION % mm mm Mm² S-StentBARE METAL 33.5% ± 9.2% 1.80 ±0.5 7.6 control S-Stent Polymer-Only 34.9%± 4.8% 2.02 ±0.8 8.5 Coated S-Stent 32.9% ± 10.1% 0.66 ±0.2 3.27 (−57%vs Polymer/Rapamycin control) 325 microGrams S-Stent 36.8% ± 8.5% 0.74±0.3 3.61 (−50% vs Polymer/Everolimus control) 325 microGrams PS StentBARE* control 10-20% 1.19 — 4.5 PS Stent Polymer-only 10-20% 1.38 — 5.0PS Rapamycin-eluting 10-20% 0.70 — 2.9 (−35.5% vs Stent* control) 166microGrams PS Rapamycin-eluting 10-20% 0.67 — 2.8 (−37.7% vs Stent*control) 166 microGrams (Slow Release) PS Rapamycin-eluting Stent*10-20% 0.75 — 3.1 (−31.1% vs 450 microGrams control)

Example 5 Preparation of Stent with High Drug Loading

As-marketed metal corrugated-ring stents (“S-stent, corrugated ringdesign, Biosensors Intl), 14.6 mm in length, were coated with anapproximately 2 micron thick layer of parylene ‘C’ primer coating usinga plasma deposition process. Parylene coated stents were placed inxylene overnight at ambient temperature. A stock poly(dl)-lactic acidsolution containing 50 μg/μL polylactic acid (PDLA) was prepared bydissolving 100 mg PDLA in 2 mL acetone.

To prepare stents containing a drug to polymer ratio of 50%, 5 mgeverolimus was dissolved in 100 μL of the PDLA stock solution. Anadditional 20 μL acetone was added to aid in dispensing the solution.The stents were removed from the xylene and carefully blotted to removesolvent. A total of 5.1 μL coating solution was dispensed onto the outersurface of each stent. The stents were dried at ambient temperature andplaced into overnight desiccation. This resulted in a total of 212 μgeverolimus contained in 212 μg PDLA per stent.

To prepare stents containing a drug to polymer ratio of 75%, 5 mgeverolimus and 33.3 μL stock PDLA solution were mixed. An additional33.3 μL acetone was added and the mixture was dissolved. Stents wereremoved from the xylene and blotted similar to above. A total of 2.8 μLcoating solution was dispensed onto the outer surface of each stent. Thestents were dried at ambient temperature and placed into overnightdesiccation. This resulted in a total of 212 μg everolimus contained in70 μg PDLA per stent.

The finished stents exhibited an approximately 5 microns-thick coatingof everolimus/PDLA, or slightly milky appearance, which was smoothlydistributed on the top and side surfaces, and firmly attached to themetal strut surfaces.

What is claimed is:
 1. A method for inhibiting restenosis at a vascularinjury site, comprising delivering to the vascular injury site, anendovascular stent having an open-lattice structure formed of linkedfilaments, and carried on the one or more filaments, a drug-releasecoating of poly-lactide polymers having a thickness of between 3-30microns, and composed of (i) 25-55 weight percent polymer substrate and(ii) 45-75 weight percent macrocyclic triene compound having the form:

where R is H or CH₂—CH₂—OH: and expanding the stent at the vascularinjury site, to bring the drug-release coating in contact with thevessel at the injury site, said coating being effective to release anamount of the compound to inhibit restenosis at the site.
 2. The methodof claim 1, wherein the filaments are poly-l-lactide and the polymersubstrate in the coating is selected from the group consisting ofpoly-l-lactide or poly-dl-lactide.
 3. The method of claim 2, wherein thepolymer substrate in the coating is formed of poly-dl-lactide having athickness between 3-20 microns.
 4. The method of claim 1, wherein thestent further includes a polymer undercoat disposed between thefilaments of the stent body and said drug-release coating.
 5. The methodof 4, wherein said polymer undercoat is formed of a polymer selectedfrom the group consisting of ethylene vinyl alcohol, parylast, silicone,a fluoropolymer, and parylene.
 6. The method of claim 3, wherein thestent further includes a parylene polymer undercoat having a thicknessof between 1-3 microns, disposed between the filaments of the stent bodyand said poly-dl-lactide coating substrate.
 7. The method of claim 6,wherein said compound is present in the coating in an amount between 50%and 75% by weight.
 8. The method of claim 1, wherein said coatingfurther includes a bioactive agent selected from the group consisting ofan antiplatelet agent, a fibrinolytic agent, and a thrombolytic agent.9. The method of claim 1, wherein the polymer substrate in the coatingis formed of poly-dl-lactide.
 10. The method of claim 1, for use wherethe vascular injury is produced during an angiographic procedure inwhich a vessel region is overstretched at least 30% in diameter.
 11. Anendovascular stent for placement at a vascular injury site, forinhibiting restenosis at the site, comprising a body having anopen-lattice structure formed of linked filaments, and carried on theone or more filaments, a drug-release coating of poly-lactide polymershaving a thickness of between 3-30 microns, and composed of (i) 25-55weight percent polymer substrate and (ii) 45-75 weight percentmacrocyclic triene compound having the form:

where R is H or is CH₂—CH₂—OH, said stent being expandable from acontracted condition in which the stent can be delivered to a vascularinjury site via catheter, and an expanded condition in which the stentcoating can be placed in contact with the vessel at the injury site,said coating being effective to release an amount of the compound toinhibit restenosis at the site.
 12. The stent of claim 11, wherein thefilaments are poly-l-lactide, and the polymer substrate in the coatingis selected from the group consisting of poly-l-lactide orpoly-dl-lactide.
 13. The stent of claim 11, wherein the polymersubstrate in the coating is formed of poly-dl-lactide having a thicknessbetween 3-20 microns.
 14. The stent of claim 11, which further includesa parylene polymer undercoat having a thickness of between 1-3 microns,disposed between the filaments of the stent body and saidpoly-dl-lactide coating substrate.
 15. The stent of claim 11, whereinsaid coating includes the compound at an initial concentration ofbetween 50 and 75 weight percent of coating.
 16. The stent of claim 11,further comprising a polymer undercoat disposed between the filaments ofthe stent body and said drug-release coating.
 17. The stent of claim 16,wherein said polymer undercoat is formed of a polymer selected from thegroup consisting of ethylene vinyl alcohol, parylast, silicone, afluoropolymer, and parylene.
 18. The stent of claim 11, wherein saidcoating further includes a bioactive agent selected from the groupconsisting of antiplatelet agents, fibrinolytic agents, and thrombolyticagents.
 19. The stent of claim 13, wherein the polymer substrate in thecoating is formed of poly-dl-lactide.
 20. The stent of claim 13 whereinfilaments are metal filaments.